This invention relates to a multilevel quadrature amplitude modulator which has an error correcting code producing unit and to a multilevel quadrature amplitude demodulator for use as a counterpart of the modulator. Such a modulator and a demodulator are particularly useful in a radio transmission system.
In the radio transmission system, the multilevel quadrature amplitude modulator uses a radio frequency band as known in the art. In the multilevel quadrature amplitude modulator, a pair of quadrature-phase carrier signals are amplitude modulated by first and second input digital signals into a multilevel quadrature amplitude modulated signal. When each of the first and the second input digital signals is an m-bit binary signal, where m is a predetermined number which is not less than three, each of the first and the second input digital signals is capable of representing at most N levels where N is equal to 2.sup.m. In this event, the multilevel quadrature amplitude modulated signal has 2.sup.2m output signal points on a phase plane which has an origin and real and imaginary axes orthogonally crossing at the origin. The output signal points are arranged uniformly in a lattice structure on a square region which has a center point at the origin and four sides parallel to the real and the imaginary axes. Such an arrangement of the output signal points is called a square arrangement. The number of the output signal points is as many as 2.sup.2m, such as sixty-four or two-hundred and fifty-six.
A certain transmission power is required for transmitting the multilevel quadrature amplitude modulated signal by a transmitter. The transmission power is dependent on a peak amplitude of the multilevel quadrature amplitude modulated signal, namely a distance between the origin and the output signal points placed at each vertex of the square region. The peak amplitude increases in proportion to an increase in the number of the output signal points. It is desirable to reduce the transmission power. In order to reduce the peak amplitude, the square arrangement is modified into an approximately circular arrangement according to signal conversion disclosed in U.S. Pat. No. 4,716,385 issued to Seiichi Noda, namely, one of the two instant applicants, et al and assigned to the instant assignee. A multilevel quadrature amplitude modulator of Noda et al patent comprises a signal conversion circuit for converting the square arrangement into the approximately circular arrangement defined by a specific region inwardly of an approximate circle. As a result of the signal conversion, a plurality of output signal points are shifted from areas of four corners of the square region so that shifted output signal points are inwardly of the specific region. When each of the first and the second input digital signals is the m-bit binary signal, the signal conversion circuit produces first and second converted signals each of which consists of parallel m bits. The first and the second converted signals are modulated into the multilevel quadrature amplitude modulated signal for carrying the first and the second converted signals as transmission data information. On carrying the transmission data information, the multilevel quadrature amplitude modulated signal has a decreased peak amplitude which is equal to a radius of the specific region and which is shorter than a half of a diagonal of the square region.
In the meanwhile, the multilevel quadrature amplitude modulated signal is inevitably subjected to a transmission error, namely, a code error, of the transmission data information. In order to reduce the code error, the multilevel quadrature amplitude modulator of Noda et al patent further comprises first and second error correcting code producing circuits each of which makes use of an error correcting code. As the error correcting code, a Lee-error correcting code is exemplified in a report contributed by Katsuhiro Nakamura, namely, by the other of the instant applicants, to ICC Conference Record, Vol. 4-3 (June 1979), pages 45.4.1 to 45.4.5, under the title of "A Class of Error Correcting Codes for DPSK Channels". The first error correcting code producing circuit is for the first converted signal consisting of parallel m bits while the second error correcting code producing circuit is for the second converted signal consisting of parallel m bits. Each of the first and the second error correcting code producing circuits carries out error correcting calculation for parallel m bits and produces first and second error correcting code sequences each of which consists of parallel m bits. In this event, it will be understood that the first and the second error correcting code producing circuits carry out the error correcting calculation for the output signal points of the square arrangement. The first and the second error correcting code sequences are multiplexed into the first and the second converted signals, respectively, in the form of redundant information. It should be understood that the multilevel quadrature amplitude modulated signal has the peak amplitude which is equal to a half of a diagonal of the square region on carrying the redundant information.